The invention relates to a cellular radio system comprising in each cell at least one base station communicating with terminal equipments located within its area, which base stations comprise means for measuring the direction angle and distance of each terminal equipment with respect to the base station, means for calculating the location of each terminal equipment in the coverage area of the base station on the basis of the direction angle and distance of the terminal equipment, and which terminal equipments comprise means for maintaining a list of nearby base stations, and means for measuring the signal strength from the base stations that are in the list kept by the terminal equipment, in order to determine the need for a handover.
The present invention is applicable for use in a data transmission system applying any multiple access method, but especially in a cellular system utilizing code division multiple access. Code division multiple access (CDMA) is a multiple access method, which is based on the spread spectrum technique and which has been applied recently in cellular radio systems, in addition to the earlier developed FDMA and TDMA methods. CDMA has several advantages over those earlier developed methods, for example spectral efficiency and the simplicity of frequency planning. An example of a known CDMA system is disclosed in the EIA/TIA Interim Standard: Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95, July 1993, EIA/TIA IS-95, which is incorporated herein by reference.
In the CDMA method, the narrow-band data signal of the user is multiplied to a relatively wide band by a spreading code having a considerably broader band than the data signal. In known test systems, bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate spreading code is used over each connection, between a respective base station and a respective mobile station, and the signals of the different users can be distinguished from one another in the receivers on the basis of the spreading code of each user.
Matched filters provided in the respective receivers are synchronized with a desired signal, which they recognize on the basis of a spreading code. The data signal is restored in the respective receiver to the original band by multiplying it again by the same spreading code that was used during the transmission. Signals multiplied by some other spreading code do not correlate in an ideal case and are not restored to the narrow band. They appear thus as noise with respect to the desired signal. The spreading codes of the system are preferably selected in such a way that they are mutually orthogonal, i.e. they do not correlate with each other.
In a CDMA cellular radio system, it is possible to use a so-called pilot channel in the transmission direction of base stations to subscribers, i.e. in the downlink direction. A pilot channel is a signal which is transmitted with a specific spreading code and utilizing the same frequency band on which the actual traffic channels are situated, the pilot signal being distinguishable from them only on the basis of the spreading code. The pilot signal is a channel known and listened to by all subscriber equipments within the cell area, and it is used, for example, in power measurements and in the generation of a coherent phase reference. Each base station of the system transmits its own pilot signal on the basis of which the subscriber equipments can distinguish the transmissions of different base stations from each other.
In a typical mobile phone environment, the signals between a base station and a terminal equipment propagate along several paths between the transmitter and the receiver. This multipath propagation is mainly due to the reflections of the signal from surrounding surfaces. Signals which have propagated along different paths arrive at the receiver at different times, due to their different transmission delays. In the CDMA system, the multipath propagation can be exploited in the reception of the signal in the same way as diversity. The receiver generally utilized in a CDMA system is a multibranch receiver structure, where each branch is synchronized with a signal component which has propagated along an individual path. Each branch is an independent receiver element, the function of which is to compose and demodulate one received signal component. In a conventional CDMA receiver, the signals of the different receiver elements are combined advantageously, either coherently or incoherently, whereby a signal of good quality is achieved.
CDMA systems can also apply a soft handover, wherein a mobile station may simultaneously communicate with several base stations by utilizing macrodiversity. The connection quality of the mobile station thus remains high during the handover and the user does not notice a break in the connection.
Interference caused in a given connection, by other connections thus appears in a receiver of the given connection as noise that is evenly distributed. This is also true when a signal is examined in an angular domain according to the incoming directions of the signals detected in the receivers. The interference caused by the other connections in the given connection thus also appears in respective receiver as distributed in the angular domain, i.e. the interference is rather evenly distributed across the different incoming directions.
The capacity of the CDMA system, which can be measured by means of spectral efficiency, has been further improved with sectorization. When sectorization is implemented, a cell is divided into sectors of a desired size that are serviced by directional antennas. The mutual noise level caused by the mobile stations can thus be reduced significantly in the base station receiver. This is based on the fact that, on average, the interference is evenly distributed across the different incoming directions, the number of which can thus be reduced by means of sectorization. The sectorization can naturally be implemented in both transmission directions. The advantage provided in the capacity by the sectorization is proportional to the number of the sectors.
A sectorized cell may also utilize a softer handover, wherein a mobile station performs a handover from one sector to another by communicating simultaneously with both sectors. Even though soft handover improves the connection quality and sectorization increases the system capacity, the movement of the mobile stations naturally leads to the stations performing several handovers from one sector to another. This loads the processing capacity of the base station controller. Several soft handovers also produce a situation where several mobile stations communicate simultaneously with more than one sector (usually, two sectors), whereby the increased capacity provided by the sectorization is lost as a signal of a mobile station is audible in a wide sector.
The multiple access interference of the CDMA systems has also been reduced by means of different known multiple access interference cancellation (IC) methods and multi-user detection (MUD). These methods are best suited for reducing the interference produced within a user's own cell, and the system capacity can thus be increased to about a double, compared to a system implemented without interference cancellation. However, these methods do not significantly improve the size of the coverage area of a base station. Also, the IC/MUD techniques are complicated to realize, wherefore they have only been developed in the uplink direction, and the opposite transmission direction is similar to that of a conventional CDMA system.
Another method that has been developed is an SDMA (Space Division Multiple Access) method wherein the users are distinguished from one another on the basis of their location. This is performed in such a way that the beams of the receiver antennas at the base station are adjusted to be directed towards the desired directions according to the locations of the mobile stations. For this purpose, the system uses adaptive antenna groups, i.e. phased antennas, and processing of the received signal, by means of which the mobile stations are tracked.
The use of SDMA in connection a CDMA system provides several advantages over the prior methods, such as sectorization. If the sector beams in the sectorization are narrowed in order to increase the spectral efficiency, the number of the handovers to be performed from one sector to another also increases. This in turn excessively increases the calculation capacity required in the base station controller.
In connection with the application of SDMA, the background art is illustrated in A.F. Naguib, A. Paulraj: Performance of CDMA Cellular Networks With Base-Station Antenna Arrays (Proc. International Zhrich Seminar on Digital Communications, pp. 87-100, Zhrich, Switzerland, March 1994), which is incorporated herein by reference. In SDMA, a signal is thus received by means of an antenna group, and the received signal is shaped by means of digital signal processing in such a way that the directivity patterns of the antennas are suitable for the stages following the shaping in the receiver. In prior art arrangements, the received signal is shaped in order to maximize the signal-to-interference ratio of the desired signal. The received signal is thus shaped in such a way that the directivity pattern of the antenna group minimizes the interference caused by the other connections in the desired signal. In the arrangement according to the aforementioned Naguib publication, each detected signal component is subjected to individual beam shaping, i.e. the impulse response must be known before the shaping is performed.
The subscriber equipments continuously measure the strength of the signal they have received from the base station. In CDMA systems, the signal measurement is generally performed as the measurement of the pilot signal. In order to reduce the measurement load of a terminal equipment, in the prior art systems each terminal equipment keeps a list of the base stations that are situated near respective terminal equipment and that are possible candidates for handover or call set-up, and of the corresponding spreading resoective pilot signals. This list is called hereinafter in this document will be called below a measurement list. The terminal equipments monitor with the highest priority the pilot signals of only those base stations that are on the measurement list. The other detected pilot signals are measured secondarily.
When a terminal equipment moves, the measurement list must be updated as the need arises. The updating is performed in the prior art systems according to the measurement of the strength of the pilot signal performed by the terminal equipment, i.e. if it is detected on the basis of the measurement of the terminal equipment that a pilot transmitted by some base station is received with sufficient strength, the base station is added in the measurement list, and correspondingly if the signal from some base station deteriorates, the base station is removed from the list.
In order to enable a fast handover, the measurement list should be as short as possible and it should contain only those base stations to the respective areas of which the terminal equipment is likely to move. In such a case, the terminal equipment can perform the measurements rapidly. The updating of the measurement list should also be fast. This is true especially in systems where the cell sizes are rather small compared to the speed of movement of the terminal equipment.
In prior art arrangements, as in the EIA/TIA IS-95 arrangements already mentioned above, the measurement list is maintained on the basis of the measured strengths of the received signals. The list cannot then be made very short, in order that no essential base stations are left out of it. Therefore, the rate of handover is not the best possible, especially with respect to small microcells.
A prior art arrangement is illustrated in FIG. 1 of the attached drawings, which shows a system comprising a group of base stations 164 to 168 the coverage areas of which are each divided into three sectors, and a subscriber equipment 102, which is situated in the area of the base station 167 and which communicates with it in a sector 162. The subscriber equipment receives not only the signal from its own base station sector 162, but also signals from the surrounding sectors. The list of each terminal equipment typically comprises most of the surrounding sectors, in the example of FIG. 1, sectors 150 to 160, and the terminal equipment continuously monitors the signals of these sectors.